Zinc-rich coatings containing conductive carbon black



May 27, 1969 N. E. KING, JR 3,446,770

ZINC-RICH COATINGS CONTAINING CONDUCTIVE CARBON BLACK Filed July 13, 1966 PROJECTIONS P-LANE EDGES WELD POCKET FIG. I

CREVICE L 1 msms CORNER/ FIG. 2 FIG. 3

NOBAL E. KING JR.

INVENTORF AGENT United States Patent US. Cl. 260-41.5 2 Claims ABSTRACT OF THE DISCLOSURE Improved corrosion resistant coatings for ferrous metals comprise 12 to 32%% by weight binder, 65 to 87% Zinc dust and 0.9 to 2.6% electrically conductive carbon black.

This invention relates to corrosion resistant coatings for ferrous metals. In particular, it relates to such coatings in the form of a paint or primer containing metallic zinc.

It is well known that metals, particularly steel, can be protected against the rapid deteriorative effects of corrosion by the elements in its environment by applying thereto a coating of metallic zinc. This protection is provided in several ways. One way is that zinc itself is much more durable under most service conditions that is steel, Thus, the zinc coating forms a barrier on the surface of the steel which corrodes only very slowly while shielding the steel from the corrodent.

Another, and equally significant, protection is the electrochemical or sacrificial action of the zinc. This effect is exterted when the steel and the zinc particles are electrically coupled and in contact with a conductive aqueous solution. In such a case, the steel is protected at the expense of the zinc since the electrical potential of the Zinc is sufiiciently higher than that of the steel (iron) to direct the flow of electrons to the steel, maintaining a negative charge on the steel surface and preventing the formation of the ferrous ions which represent rust. This effect re sults in the formation of zinc hydroxide on the zinc which in turn reacts with chlorine or carbon dioxide in the surrounding environment to form basic zinc salts. These basic zinc salts plate out on the bare steel surface to form a protective coating which affords good barrier protection to the steel. When this effect is taking place, the metallic zinc is, of course, being consumed by the salt formation; thus the term sacrificial action. The typical case where this process comes into play is that where a steel structure is coated with zinc and a scratch or other discontinuity develops in the coating. The sacrificial action then continues until the damaged area has been sufficiently plated to yield protection. In effect, the zinc acts as the anode of a galvanic cell while the steel acts as the cathode. This effect is well known as the source of the expression galvanized iron.

The application of a zinc coating directly to steel is a rather cumbersome and expensive operation. In the past this has been accomplished by hot dip coatings or electroplating, both of which are to some extent objectionable for the above reasons.

More recently, it has become the practice to coat the metal item sought to be protected with a paint containing a sufiicient amount of metallic zinc to impart the desired resistance to corrosion. These paints have proven very effective in providing both the cathodic or galvanic type protection and the barrier type as well. However, in order to provide the desired degree of cathodic protection, a certain minimum electrical conductivity of the coating solution is required. In order to provide the required level of conductivity, a very high level of zinc in the coating is required. In most cases this is as high as to by weight of the total solids content of the coating.

The previously known zinc containing coating compositions suffer at least two major shortcomings as a result of the high zinc loading. For one, the high zinc content pushes the cost of these coatings to an undesirable level. Even more troublesome, however, is the fact that the low level of binder material relative to that of the metallic zinc results in the formation of relatively weak films which disintegrate or degrade rather readily when subjected to even normal usage.

In accordance with the instant invention, coating compositions containing zinc are provided which exhibit the same high level of cathodic protection against corrosion as is shown by prior art compositions, but which do so at greatly reduced zinc loading. These compositions comprise a film-forming binder or vehicle, metallic zinc, and conductive carbon black. More specifically, these compositions comprise about 12 to 32.5% by weight binder, based on total weight of components, 65 to 87% by weight metallic zinc, and 0.9 to 2.6% by weight conductive carbon black.

Whereas, as previously noted, prior art compositions of this nature, in order to be effective, required zinc loadings as high as 85 to 95% by weight, drastically reducing the allowable binder content therein, the total content of Zinc and carbon black combined need not exceed 85% in the instant compositions. Thus, these compositions contain sufficient binder to impart a high level of strength and durability to the coatings which was not always the case with prior art coatings.

The compositions of this invention, when used as prime coatings, have been found to be less prone to cause bubbling in a subsequently applied topcoat than are the more highly loaded compositions of the prior art. This is likewise believed to be closely related to the lower pigment concentration in these compositions.

Compositions according to this invention can be prepared using any of the binders normally used in preparing corrosion resistant coatings or outdoor coatings generally. Such binders include chlorinated rubber, polyisoprene, poly(vinyl acetate), vinyl chloridevinyl acetate copolymers, polyurethanes, catalyzed epoxy and epoxyester resins, chlorinated polyolefins, alkyd resins and inorganic binders, to name a few. The type of binder depends to some extent upon the type of service anticipated. A particularly good binder, and a preferred one, is chlorinated rubber containing about 63 to 69% chlorine. This particular binder is effective under a wide variety of conditions and is quite extensively used due to its environmental stability and durability.

Normally the coating compositions according to this invention are employed as primers or undercoatings under one or more finish coats. Their utility is, of course, not limited to this application, although the coloration due to the carbon black may limit their utility in many cases. Where the black color is not a deterrent, these compositions can be employed as finish coats if they are otherwise properly formulated.

The metallic Zinc in these compositions is in small particle form. These particles can be in the form of extremely small flakes having thickness between about 0.1 and 0.5 which can be prepared by atomizing extremely pure zinc (about 99.7% Zn or more) followed by ball milling. Equally preferable are spherical particles having diameter between about 0.1 and 0.5,. These can be prepared by distillation and subsequent condensation of the high purity Zn metal. No further treatment of the zinc is required prior to incorporation thereof into the coating composition.

Any of the commercially available electrically conductive carbon blacks can be employed. Preferably, the black is made from the thermal degradation of acetylene and will have a particle size between about 0.005 and 0.20011 A particularly useful black is that known as acetylene black, prepared :by thermal decomposition of acetylene without combustion thereof.

'4 Aluminum powder was in flake form. Flakes were approximately 0.13;t thick. Not less than 90% of the flakes will pass through a 325 mesh standard screen having 44p. openings.

For application to a substrate, the comp n are The formulations were prepared by pebble milling the incorporated into a llqurd medium which is, in most cases, mixture lfol. 8 hours.

a solvent for the binder material and a suspending medium Standard Steel test panels of the type Shown in the 9 Zmc carbon l A mm of the bmfier mate' drawing were sandblasted to white metal and spray coated rial with the p1gments dispersed therethrough is formed with the two formulations to a thickness of about 2.5 to by evaporation of the solvent llquid. In some cases, the 10 3 5 Th th d hl t d binder material can be an uncured or partially polym- 2' 686 ig j a i h erized resin which forms a film by completing its polymer to a t o to S an t en wit erization after the solvent or carrier has evaporated. In a T102 Pigmented chlonnated rubber topcoat of about either case, the selection of the liquid medium depends 2 to 3 upon the binder to be used. Selection of the binder-liquid 15 A thlfd Set of test Panels Was treated Wlth a m combination to be used is within the skill oi the art and cial zinc rich primer believed to be comprised of zinc forms no part of this invention. particles suspended in a sodium silicate binder and over- Generally speaking, the compositions of this invention coated i i1 fl are otherwise comprised of combinations of ingredients The coated test panels were exposed to a highly corm to i g various plasticizers known sive industrial atmosphere of a plant in Brunswick, 6a., to be effective or desirable with the selected binder mateh m d h d d Af h rial are frequently used, as well as additional resins, to W are c Oflnat? Camp ene 15 pro uce ter six mont 5 yield specific properties desired. It is also sometimes desirf to thl s CQITOSlVe mosphere, the panels were able to incl de Wetting agents t i t i suspending h inspected for evidence of rust and corros1on. A quahtatrve solid, insoluble pigments, as well as additives, to modify and quantitative evaluation of each panel at each of the the drying rate or curing rate of the composition following i i al spots as made as follows: its application to a substrate.

The coating compositions of necessity must exhibit a Good (G).=no apparent change in coatingzz points certam mmnmmi 'conduciwlty m order to i i the Fair (F)=some change, 5% of surface attacked==1 necessary cathodic protection to the metal WhlCh Is to be point coated therewith. The conductivityisafunctlon of the total Bad (B) film f has 0c r d t concentration of zinc and carbon black in the composition t a1 re cu re ms appeare and the degree of interdispersion of the two. Thus, the pom carbon black and the zinc must be intimately mixed I throughout the composition during the preparation thereof. The ratlnss of the three coatings were s m e fo l w- Such mixing is readily accomplished by known methods ing table.

Area Overall rating Inside Individual Coating Panel No. Projection Plane Edge Pit Weld Crevice corner Pockets panels Composite Control A 1 G G G G B B F B 9 17 2 G G F F F B F B s ControlB 1 G G B G G B G F 11} 22 2 G G B G G B F G 11 Examplel 1 G G G G G B G G 14} 29 2 G G G G G F G G 15 of mixing. Normally, the composition, including the 501- The overall number ratings clearly show that the test vent, is passed through a conventional paint mill or the panels primed with the compositions of this invention like to eitect intimate mxmg. resist the corrosive eifects of the atmosphere better than The S01V6Ilt-f0nta1fl111g mat61'1={1 can be PP tofhe those coated with either of the compositions which do metal by spraying, brushing, or in any other convenient not contain carbon black The Scoslty or flow charactenstlcs of the 9 Substantially similar results were achieved when other wmfidepend upon the method to be used for applying the test panels containing these same coatings were subjected coa to a tidal exposure in Miami, Florida for six months.

In the attached i 2 3 1nuStr.a.te Again, the best corrosion resistance was exhibited by standard test panel used in exposing coating compositions those anels which re rim d th to the atmosphere. The critical points of exposure are i a We p e W1 e composl Ions designated on the panel. This type panel was employed in o mven the testing of the formulation of Example 1. Exa m P16 2 Example 1 A zinc rich primer containing carbon black and a con- A senes g z g tiased g ifi trol primer were prepared having the following formulawas pliepare 9 emons ta 5 e Y sacr} or tions: galvanic protection aiforded by the 21110 l'lCh coatings containing carbon black. In each case, the film-forming ve- Oonml 1 hicle was composed of:

2.12 7.19 1.16 3. 91 Parts 23 Chlorinated rubber 53.0 M owder g-g Chlorinated blphenyl 14.2 Org anic mo'uiiiilili jfionifiofiiiniifijiiil I42 C lo inated triphenyl 28.4 gggg ggggggeg 'f 1?,2 Organic o d magnesium Xylene 17.60 27.00 montmorillonlte 2.6 VM & P naphtha..-" 4.40 s. on

Dibasic lead phosphate 1.8

Film integrity Percent elongation Specimen eatazccewenwwwww The data clearly show that the compositions of relatively 10w zinc content containing carbon black (A through F) have definitely improved physical properties over those (G through L) having the higher zinc loadings required of prior art zinc rich corrosion resistant coatings. Coatings G through L were observed to be brittle, flaking or powdering when scratched. Their performance was characteristic of coatings containing too little binder for adequate film cohesion.

Example 4 The coatings described as A, B, C and D in Example 2 above were applied in triplicate to sand-blasted 3 x 5 inch steel panels having an inverted V-shaped area masked out so as to remain unpainted according to Steel Structures Painting Council Paint Specification 12-64P. The masked out area runs from top to bottom of the panel, tapering from /2 inch in width at one end to 0 at the other.

To test the corrosion resistance of panels thus coated, the panels were subjected to 96 hours salt fog exposure.

The efficacy of the coating is judged by the degree of rust build up; the coated panel should show no rust on the top 1 inch of the masked out area or on the coated area, but should show moderate rust 1 inch from the bottom of the masked out area.

In this test, both of the zinc rich primers containing carbon black passed, both without carbon black failed in that they showed considerable rust in the upper 1 inch of the masked out area.

What I claim and desire to protect by Letters Patent is:

1. A corrosion resistant metal coating composition having a non-volatile phase comprising about 12 to 32.5% of binder by weight, to 87% by weight of high purity, metallic zinc in small particle form and 0.9 to 2.6% by weight of electrically conductive carbon black in the form of 0.005 to 0.2 micron particles.

2. The composition of claim 1 where the binder material is chlorinated rubber containing a minimum of 63% chlorine.

References Cited UNITED STATES PATENTS 3,117,883 l/1964 Gilchrist 106290 3,234,038 2/ 1966 Stephens et a1. l06290 3,242,001 3/ 1966 Lucas. 3,356,515 12/1967 McGlothlin l06290 MORRIS LIEBMAN, Primary Examiner.

S. L. FOX, Assistant Examiner.

U.S. Cl. X.R. 

